Transparent resin films are used in a variety of optical and display applications. In particular, resin films are used as protective cover sheets for light polarizers in a variety of electronic displays, particularly Liquid Crystal Displays (LCD).
LCDs contain a number of optical elements that may be formed from resin films. The structure of reflective LCD's may include a liquid crystal cell, one or more polarizer plates, and one or more light management films. Liquid crystal cells are formed by dispersing liquid crystals such as twisted nematic (TN) or super twisted nematic (STN) materials between two electrode substrates. Polarizer plates are typically a multi-layer element of resin films and are comprised of a polarizing film sandwiched between two protective cover sheets. Polarizing films are normally prepared from a transparent and highly uniform amorphous resin film that is subsequently stretched to orient the polymer molecules and stained with a dye to produce a dichroic film. An example of a suitable resin for the formation of polarizer films is fully hydrolyzed polyvinyl alcohol (PVA). Because the stretched PVA films used to form polarizers are very fragile and dimensionally unstable, protective cover sheets are normally laminated to both sides of the PVA film to offer both support and abrasion resistance. Protective cover sheets of polarizer plates are required to have high uniformity, good dimensional and chemical stability, and high transparency. Originally, protective cover sheets were formed from glass, but a number of resin films are now used to produce lightweight and flexible polarizers. Although many resins have been suggested for use in protective cover sheets including, cellulosics, acrylics, cyclic olefin polymers, polycarbonates, and sulfones, acetyl cellulose polymers are most commonly used in protective cover sheets for polarizer plates. Polymers of the acetyl cellulose type are commercially available in a variety of molecular weights as well as the degree of acyl substitution of the hydroxyl groups on the cellulose backbone. Of these, the fully substituted polymer, triacetyl cellulose (TAC) is commonly used to manufacture resin films for use in protective cover sheets for polarizer plates.
The cover sheet normally requires a surface treatment to insure good adhesion to the PVA dichroic film. When TAC is used as the protective cover film of a polarizer plate, the TAC film is subjected to treatment in an alkali bath to saponify the TAC surface to provide suitable adhesion to the PVA dichroic film. The alkali treatment uses an aqueous solution containing a hydroxide of an alkali metal, such as sodium hydroxide or potassium hydroxide. After alkali treatment, the cellulose acetate film is typically washed with weak acid solution followed by rinsing with water and drying. This saponification process is both messy and time consuming. U.S. Pat. No. 2,362,580 describes a laminar structure wherein two cellulose ester films each having a surface layer containing cellulose nitrate and a modified PVA is adhered to both sides of a PVA film. JP 06094915A discloses a protective film for polarizer plates wherein the protective film has a hydrophilic layer which provides adhesion to PVA film.
Some LCD devices may contain a protective cover sheet that also serves as a compensation film to improve the viewing angle of an image. Compensation films (i.e. retardation films or phase difference films) are normally prepared from amorphous films that have a controlled level of birefringence either by uniaxial stretching or by coating with discotic dyes. Suitable resins suggested for formation of compensation films by stretching include polyvinyl alcohols, polycarbonates and sulfones. Compensation films prepared by treatment with dyes normally require highly transparent films having low birefringence such as TAC and cyclic olefin polymers.
Protective cover sheets may require the application of other functional layers (herein also referred to as auxiliary layers) such as an abrasion resistant hardcoat layer, antiglare layer, antireflection layer, anti-smudge layer, tie layer, barrier layer, or antistatic layer. Generally, these functional layers are applied in a process step that is separate from the manufacture of the resin film.
In general, resin films are prepared either by melt extrusion methods or by casting methods. Melt extrusion methods involve heating the resin until molten (approximate viscosity on the order of 100,000 cp), and then applying the hot molten polymer to a highly polished metal band or drum with an extrusion die, cooling the film, and finally peeling the film from the metal support. For many reasons, however, films prepared by melt extrusion are generally not suitable for optical applications. Principal among these is the fact that melt extruded films exhibit a high degree of optical birefringence. In the case of highly substituted cellulose acetate, there is the additional problem of melting the polymer. Cellulose triacetate has a very high melting temperature of 270-300° C., and this is above the temperature where decomposition begins. For these reasons, melt extrusion methods are generally not practical for fabricating many resin films including cellulose triacetate films used to prepare protective covers and substrates in electronic displays. Rather, casting methods are generally used to manufacture these films.
Resin films for optical applications are manufactured almost exclusively by casting methods, see for example U.S. Pat. No. 4,895,769 to Land and U.S. Pat. No. 5,925,289 to Cael. Casting methods involve first dissolving the polymer in an appropriate solvent to form a dope having a high viscosity on the order of 50,000 cp, and then applying the viscous dope to a continuous highly polished metal band or drum through an extrusion die, partially drying the wet film, peeling the partially dried film from the metal support, and conveying the partially dried film through an oven to more completely remove solvent from the film. Cast films typically have a final dry thickness in the range of 40-200 microns. In general, thin films of less than 40 microns are very difficult to produce by casting methods due to the fragility of wet film during the peeling and drying processes. Films having a thickness of greater than 200 microns are also problematic to manufacture due to difficulties associated with the removal of solvent in the final drying step. Although the dissolution and drying steps of the casting method add complexity and expense, cast films generally have better optical properties when compared to films prepared by melt extrusion methods, and problems associated with decomposition at high temperature are avoided.
Birefringence in cast or coated films arises from orientation of polymers during the manufacturing operations. This molecular orientation causes indices of refraction within the plane of the film to be measurably different. In-plane birefringence is the difference between these indices of refraction in perpendicular directions within the plane of the film. The absolute value of birefringence multiplied by the film thickness is defined as in-plane retardation. Therefore, in-plane retardation is a measure of molecular anisotropy within the plane of the film.
During a casting process, molecular orientation may arise from a number of sources including shear of the dope in the die, shear of the dope by the metal support during application, shear of the partially dried film during the peeling step, and shear of the free-standing film during conveyance through the final drying step. These shear forces orient the polymer molecules and ultimately give rise to undesirably high birefringence or retardation values. To minimize shear and obtain the lowest birefringence films, casting processes are typically operated at very low line speeds of 1-15 m/min as disclosed in U.S. Pat. No. 5,695,694 to Iwata. Slower line speeds generally produce the highest quality films.
Another drawback to the casting method is the inability to accurately apply multiple layers. As noted in U.S. Pat. No. 5,256,357 to Hayward, conventional multi-slot casting dies create unacceptably non-uniform films. In particular, line and streak non-uniformity is greater than 5% with prior art devices. Acceptable two layer films may be prepared by employing special die lip designs as taught in U.S. Pat. No. 5,256,357 to Hayward, but the die designs are complex and may be impractical for applying more than two layers simultaneously.
Another drawback to the casting method is the restrictions on the viscosity of the dope. In casting practice, the viscosity of dope is on the order of 50,000 cp. For example, U.S. Pat. No. 5,256,357 to Hayward describes practical casting examples using dopes with a viscosity of 100,000 cp. In general, cast films prepared with lower viscosity dopes are known to produce non-uniform films as noted for example in U.S. Pat. No. 5,695,694 to Iwata. In U.S. Pat. No. 5,695,694 to Iwata, the lowest viscosity dopes used to prepare casting samples are approximately 10,000 cp. At these high viscosity values, however, casting dopes are difficult to filter and degas. While fibers and larger debris may be removed, softer materials such as polymer slugs are more difficult to filter at the high pressures found in dope delivery systems. Particulate and bubble artifacts create conspicuous inclusion defects as well as streaks and may create substantial waste.
In addition, the casting method can be relatively inflexible with respect to product changes. Because casting requires high viscosity dopes, changing product formulations requires extensive down time for cleaning delivery systems to eliminate the possibility of contamination. Particularly problematic are formulation changes involving incompatible polymers and solvents. In fact, formulation changes are so time consuming and expensive with the casting method that most production machines are dedicated exclusively to producing only one film type.
More recently, commonly-assigned U.S. Patent Application Publications 2003/0215658A, 2003/0215621A, 2003/0215608A, 2003/0215583A, 2003/0215582A, 2003/0215581A, 2003/0214715A and U.S. patent application Ser Nos. 10/838,681 and 10/838,841 filed May 4, 2004 describe peelable resin films that are applied by coating onto a temporary carrier substrate using lower viscosity polymer solutions than are normally used to prepare cast films. The adhesion of the peelable resin film to the discontinuous, temporary carrier substrate must be carefully designed. If the peelable film is only weakly adhered to the carrier substrate, the film may be prone to premature delamination from the carrier substrate during its manufacture or during subsequent coating steps. This may thus result in a reduction in the efficiency of the manufacturing/coating facility. Premature release during manufacture can be avoided by making the peelable film strongly adherent to the carrier substrate, but at the cost of poor peeling of the releasable film from the carrier substrate by the end user.
What is needed is an improved optical film composite that reduces the risk of premature delamination of the optical film during manufacture of the composite while allowing easy peeling of the optical film during customer use.